On Fri, 2010-11-19 at 12:50 -0500, J. Bruce Fields wrote: > On Thu, Nov 18, 2010 at 03:31:10PM -0800, Linus Torvalds wrote: > > On Thu, Nov 18, 2010 at 3:02 PM, Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> wrote: > > > > > > This patchset separates the incrementing/decrementing of the i_readcount, in > > > the VFS layer, from other IMA functionality, by replacing the current > > > ima_counts_get() call with i_readcount_inc(). Its unclear whether this call to > > > increment i_readcount should be made earlier, like i_writecount. Currently the > > > call is situated immediately after the switch from put_filp() to fput() for > > > cleanup. > > > > Well, it seems nicer than the situation we have now. So I'm certainly > > ok with seeing this merged for 2.6.38 (through the security tree?) if > > nobody has objections. > > > > It's a bit sad to have another atomic in the open path, but if the > > lease people want this and are ok with just the counter (no races?) > > then it seems worth it. > > Having thought about it more, I'm no longer convinced it will be useful > for leases. > > It seems attractive to replace the current d_count/i_count checks by an > i_readcount check, but: > > 1) as long as break_lease() is called before i_readcount_inc(), > there's a window between the two where setlease has no way to > tell whether a read open is about to happen; > > 2) more importantly, it won't help file servers, which need more > than mutual exclusion between opens and leases. > > Number 2 in more detail: > > Write leases exist to let a file server (nfsd or Samba) tell a client > that it has exclusive access to a file, so that the client can delay > writes, knowing that it will be notified on lease break (and given a > chance to write back dirty data) before someone else can look at the > file. > > But say someone modifies a file on a client and then runs "make" on the > server. The "make" needs to know about the modifications. But make only > stat's the file, doesn't open it.... Hi Bruce, IMA (and the proposed EVM/IMA-appraisal patches) detects file change based on i_version. When the file is closed, if the file has changed, IMA marks the file as needing to be re-measured. Of course this requires the filesystem to be mounted with iversion. Don't know if this helps. Mimi > We can break leases on stat, but on its own that's racy--setlease needs > some way to determine whether a lease is in progress. And i_readlease() > doesn't help there, unless we decide we're going to temporarily > increment that around every stat. (But if another atomic in the open > path is bad, another in the stat path sounds worse--and it's probably > not the semantics ima needs anyway.) > > --b. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
